HarpinPss, a proteinaceous elicitor from Pseudomonas syringae pv. syringae, is a glycine-rich, cysteine-lacking, heat-stable protein. It can elicit the hypersensitive response (HR) when delivered to the surface of plant cells. HRAP (hypersensitive response assisting protein) is an amphipathic protein purified from sweet pepper and could intensify harpinPss¡Vmediated HR in sweet pepper. In the previous research, harpinPss was present as monomer, dimer, trimer, tetramer, and ocatamer forms in neutral pH buffer. Only monomer and dimer forms of harpinPss induced hypersensitive response in nonhost plants. HRAP could cause multimeric forms of harpinPss dissociation into monomer forms. The interaction between HRAP and harpinPss is an important issue. HRAP contained three positively charged regions, a typical signal peptide and a cAMP-dependent phosphorylation site. In this study, these regions of HRAP would be truncated and identified whether these truncated HRAP fragments could promote harpinPss dissociation. Different combinations of truncated HRAP and harpinPss were used to identify the protein-interaction regions between two proteins. HarpinPss triggers HR via interaction with cAMP phosphorated region of HRAP and MAPK pathway transduction. When cAMP region of HRAP was truncated, harpinPss still triggers HR via polymerization and anchor on lipid bilayers to form an ion-conducting pore.

Harpinpss, a pathogenic protein, encoded by hrpZ in the hrp gene cluster from Pswudomonas syringae pv. syringae, can induce the hypersensitive response in tobacco (Nicotiana tabacum L. cv. Xanthi). The lesion area on the tobacco leaves was visible 6 h after inoculation with harpin, and was evident 12 h after inoculation. The lignin content in harpin-treated tobacco leaves was about 2.5-fold as compared with the controls 24 h after inoculation. There were six isozymes of POD (pI 9.5, pI 8.7, pI 5.3, pI 4.4, pI 3.7, and pI 3.5) and seven isozymes of laccase (pI 9.4, pI 8.6, pI 7.8, pI 5.4, pI 4.5, pI 3.8, and pI 3.6) identified by isoelectric point in extracts of harpin-inoculated tobacco leaves. POD isozymes (pI 4.4, pI 5.3 and pI 8.7) and laccase isozyme (pI 7.8) only appeared in harpin-inoculated tissues. The increased POD isozymes (pI 4.4, pI 8.7, pI 9.5) are correlated with the rise of transcripts of these enzymes confirmed by the method of reverse transcriptase-polymerase chain reaction (RT-PCR).